For numerous purposes containers of plastic materials are used, e.g. for containing liquids such as, for example, beverages and the like.
Such beverages frequently contain carbon dioxide or contain gases which can be released if the temperature is varied or the liquids are agitated, thus raising the pressure in the container.
Containers suitable for this purpose must, therefore, be resistant to the internal pressure applied to them and, moreover, must be impermeable over periods of time to the gases dissolved in the contained liquids.
A plastic material suitable for this purpose is polyethylene terephthalate, known by the abbreviation PET, which may be subjected to mechanical drawing deformations at an appropriate temperature. Such which deformations impart to the plastic material an oriented molecular structur increasing mechanical strength and imparting to gases substantially better then those of the non-oriented polymer and suitable for use in the aforementioned containers. For this purpose it is especially advantageous to provide a molecular orientation in two orthogonal directions in the material, by which the best results of mechanical strength and impermeability to gases are achieved, the mechanical strength and impermeability being of fundamental importance to the containers for liquids comprising gases in solution.